The present invention relates generally to video special effects generators for combining multiple video input signals into a single video output signal, and more particularly, to a special effects generator utilizing digital circuitry for providing a gradual transition between two video input signal switched into the video output signal.
Digital special effects generators are known. One such digital special effects generator is described in U.S. Pat. No. 3,821,468 issued June 28, 1974 to Albert E. Busch, assigned to the same assignee as the assignee of the present invention and incorporated herein by reference. While digital special effects generators such as the one described in the above-mentioned Busch reference provide a variety of special effects, transitions in the video output signal between two video input signals are abrupt, thereby resulting in a video display wherein the two video signals are displayed adjacent to each other with a sharp line separating the two displayed images. Such sharp transitions are desirable for many special effects; however, for other special effects such as vignette-type inserts, it is desirable to have a gradual transition between the two video signals to provide a soft edge effect in which the two video signals gradually blend into each other when displayed on the screen.
Circuits for providing a gradual transition between video signals have been constructed. Such circuits utilize standard special effects generators that have been modified by adding an analog video switch that gradually increases the amplitude of one of the video signals while simultaneously decreasing the amplitude of the other video signal to provide the soft edge effect. A delay circuit is generally interposed between the analog switch and the circuitry driving it to provide a gradually changing drive signal to the analog video switch.
While this technique provides a way of achieving a gradual transition between two video signals, when a symmetrical effect such as a circle or a diamond effect is selected and a soft edge is desired, the switching delay introduced by the delay circuit driving the analog video switch causes the display of the central image (i.e., the diamond or circle) to shift in the direction of scanning by an amount equal to the width of the transition border. Unless compensated, such a shift can be annoying to the viewer, particularly when a relatively wide transition border is generated.